High vacuum pump



Feb. 18, 1969 F. x. EDER ,4

HIGH VACUUM PUMP Filed May 16, 1967 Sheet of 5 INVENTOR.

Feb. 18, 1969 F. x. EDER 3,428,241

HIGH VACUUM PUMP Filed ma 16. I967 Pia. 2 i5 NVENTOR.

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INVENTOR.

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BY R 2 5 7w'* United States Patent 7,362/ 66 U.S. Cl. 230-69 Int. Cl. F04b 39/00; F041 11/00 3 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a high vacuum pump capable of producing ultra-high vacuums combining known pumping principles associated with ion, getter and cryo type pumps in a manner to enhance and improve the net pumping capability and self sufficiency. The pump is comprised of a pair of electrodes having a high voltage impressed thereacross, producing a glow discharge, ionizing the gas to be pumped. An associated magnetic coil produces a field suitably aimed, forcing the electrons to a longer mean free path, producing a high ionization probability. The invention is particularly characterized by supercooling one electrode as by a liquified gas and forming the surface of the other electrode with a getter material. The magnetic coil is also immersed in liquified gas and thus has superconducting properties.

Background For the production of very high vacuums of 10* mm. and better, it is the present practice to use diffusion pumps with cooled bafiles, getter ion pumps and cryopumps. In certain applications requiring very high pumping speeds and large vacuums, such as thermonuclear fusion experiments, for example, diffusion pumps are not adequate. Such pumps are limited in that they pump gas proportional to the thermal molecular velocity with one order of magnitude smaller speed. Effective baffles are required which strongly reduce pumping speed.

Ion getter pumps have two electrodes connected to a. high voltage of between lkv. between which a glow discharge takes place which ionizes the gas to be pumped, Through a suitably-aimed magnetic field, the electrons in the glow discharge are forced to a longer mean free path, producing a higher ionization probability. The cathode consists of a highly effective getter material such as zirconium, molybdenum or titanium and is evaporated through the impacting positive ions whereby the getter material condenses on the anode and absorbs large amounts of gases. Certain disadvantages of an ion getter pump are that they are considerably less effective in pumping inactive and noble gases, require other pumping methods, mostly diffusion pumps since the active getter surfaces have to be produced at very low pressures, and, for effectiveness, require a high strength magnetic field.

Cryopumps contain condensation surfaces held to very low temperatures, Their effect relies on the fact that the gas molecules which impact on the condensation surfaces are held with a certain probability and are condensed into a continuous surface. Nitrogen can be pumped to approximately 10 mm. on a condensation surface cooled to the temperature of liquid helium. For hydrogen, the sticking coefiicient is small at 42 K. To pump this gas effectively, it is necessary to cool the condensation surface by helium to at least 2.5 K. Helium, of course, cannot be pumped by this method at all.

The present invention avoids the disadvantages of the described known pumping methods which are especially 3,428,241 Patented Feb. 18, 1969 apparent in the ultra-high vacuum field and it makes possible the production of a very high vacuum in chemical active, as well as chemical inactive and noble gases.

These and other objectives will become apparent on consideration of the appended description and drawings wherein:

FIGURES 1 and 2 show a first form of a getter ion pump with a cooled anode in the form of a closed cool bath which contains a superconductant coil for the production of a strong magnetic field;

FIGURE 3 shows a second modification with a multisection anode container and with a system of radiation shields surrounding the pump system;

FIGURE 4 shows a cylindrical form of the pump according to the invention in which the outside mounted cathode can be optionally cooled to amplify the ion trapping effect; and

FIGURE 5 shows a pump arrangement with a centrally-located cooled container which has hollow radial ribs to receive flat superconductant coils which produce a circular one directional magnet field.

FIGURES 1a, 2a, 3a, and 5a, are section views taken along lines A-A in FIGURES l, 2, 3, and 5, respectively.

The high vacuum pump shown in FIGURES 1 and 1a consists of the liquified gas container or pump case 1 which with the aid of tube 2 protrudes downwardly through the wall into the high vacuum apparatus. The pump container 1 Whose outer wall, together with the ring-shaped discs 4 connected to this wall, form the anode of the getter ion pump and is surrounded by a cathode 5 formed from getter material such as titanium, molybdenum or zirconium. The anode consists of nonmagnetic material, such, as for example, stainless steel, and is fully suspended in the vacuum chamber by a thinwalled tube 2. The cathode 5 is formed of sheet metal sheets which are carried by the electrical leads 6 and 7 which lead through the vacuum chamber wall and allow the connection of a very high DC potential between them and the grounded pump case 1. Within the pump case 1, the magnet coil 8 made from superconductant wire or ribbon is mounted and is connected to an external power supply through leads 9 and 10 running through tube 2.

After filling with cooling liquid, preferably liquid helium, a very low pressure of approximately 10 mm. is produced in the vacuum chamber after a very short period of time. A high DC voltage is then connected between the cathode 5- and the grounded pump case 1 with the anodes 4 connected to it. At the same time, the magnetic coil 8 is excited. A glow discharge will burn between the named electrodes which because of the strong magnetic field produced by the magnetic coil, will not extinguish at even very low pressures. The ions produced by electron impact will hit the sheets 5 made from getter material and evaporate from their surfaces said getter material. This getter material condenses on the cold anode and the thus-produced active metal film of high atomic disorder absorbs the gas to be pumped at a high rate. Ionized noble gas atoms under the influence of the high electrical field strength of the cathode surface are also trapped and thereby removed. Through additional cooling of the getter sheets 5, the latter effect can be amplified. The FIGURE la shows in cross-section along the line AA- of FIGURE 1 how the cathode can be divided up into segments by ribbing it to increase the effective surface area.

The great advantage resulting from this arrangement consists of the fact that this pump works as a cryopump without applied voltage and produces a pressure of 10* mm. for nitrogen or oxygen. As voltage is applied, starting the ion getter pump, a glow discharge is produced and material is evaporated from the cathode and deposited on the anode. The effectiveness of the getter material deposited on the anode is greatly increased at the very low temperature and is adequate to pump even hydrogen as well as the expected chemically-active gases such as nitrogen, oxygen, CO CO and water vapor. It is known that the getter efiect is strongly dependent on the structure of the metal deposition formed on the anode and is especially good if the anode is amorph or even atomic rough. The cooling of the anode, or both electrodes, to very low temperatures, according to this invention, promotes the forming of such amorph or atomic rough surfaces unusually and noticeably amplifies the getter pump,

effect.

Another important aspect is the additional use of the liquified gas bath to cool the superconductant coil 8 to produce, without losses, a high magnetic field in the space between electrodes. The pumping speeds increase almost proportionally to the magnetic field strength which may be from 10,000 to 20,000 ga-uss with the aid of superconductant coils. Further, it is only necessary to supply start-up current. A continuous current can be maintained in a superconductant magnetic coil according to known methods.

Another practical example of the invention is shown in FIGURES 2 and 2a. In this case. the stray field of a short superconducting cryocoil 11 is used to strongly increase the impact probability of the electrons in the glow discharge burning between the anode 12 and the cathode 13. For this purpose, the superconducting coil 11 is mounted preferably in a cylindrical container whose outer wall forms the mentioned anode and which is filled with liquid helium. The container, in turn, is connected through metal tube 15 made from a poor thermal conductor to the wall 16 of the vacuum chamber to be evacuated. The metal tube 15 at the same time serves as filler tube for the container. To increase the magnetic field strength of the stray field of the cryocoil 11, round shield plates 17 are fastened to the bottom and the cover of the helium container which consists of a superconducting material with high critical field strengths such as, for example, Nb-Zr or Nb-Ti alloys. Through thermal contact of these shields with the helium bath, they are held in a superconducting condition, preventing the penetration of magnetic flux into their insides and thereby concentrating the stray field into the area between anode 12 and cathode 13. This is especially effective for enhancing the glow discharge and getter effect. The cathode 13 consists of getter material such as, for example, titanium, molybdenum and has the shape of diagonally-arranged single sheets which offer the gas to be pumped low resistance and also keep getter atoms from flying to the outside. These are rather supposed to condense on anode 12 which is made up as a tube section consisting of stainless steel or similarily suitable material forming the containers outer wall. The getter effect of this constantly-renewed surface is strongly increased through the effect of the very low temperature which the anode has assumed. The cathode 13 which is provided with a large number of ribs is concentrically arranged around the anode 12 and is connected to a relatively-high negative voltage in respect to the grounded anode.

Another example is shown in FIGURES 3 and 3a in which the arrangement is also mounted around a centrallylocated cooling container connected electrically as anode. The latter, however, consists of three container sections 21, 22 and 23 which, as shown in the drawing, are connected together. Each of these containers 21, 22 and 23 is provided with its own cryocoil 24, 25 and 26. The cathodes 27 and 28 are mounted in the spaces between the anodes and are connected through leads 29 to a negative high voltage. The whole arrangement is surrounded by angle-shaped radiation shields '30 which allow the penetration of the gases to be pumped, however, shield against direct heat radiation. In cases where the anode container is supplied with liquid helium, it is recommended to provide a cooling with liquid air for the radia tion shields or to connect them through thermal contact with the support of the deep cooled anode container to an intermediate temperature. The described pump is sup ported by a flange 31 and is connected through that opening to the vacuum chamber so that it protrudes into the latter. The FIGURE 3a shows a cross-section according to the line A-A of FIGURE 3 and shows the design of the cathode especially well.

A cylindric symmetrical design example is shown in FIGURE 4. To the cylindrical pump body 41 is mounted the cylindrical cathode 42 in an electrically-insulated fashion which is surrounded on its outer side by the getter metal in the form of a sheet jacket 43. The anode consists of a ring container 44 made out of nonmagnetic material which is mounted thermally insulated through two thin walled tubes 45 and 46 to the cover of the pump body. The tubes at the same time serves as (filler and exhaust tube for the liquid helium. In the ring container 44 is mounted the magnet coil 47 made of superconducting material whose connector leads run through the filler tubes. With the aid of the flange 48, the pump is connected to the vacuum chamber. The system consisting of a ring anode and a centrally-located cathode can also be mounted directly into the vacuum chamber without the pump body 41. The cathode 42 may optionally be cooled along with the anode to amplify the ion-trapping effect.

The function of this pump is exactly the same as that of the one shown in FIGURE 1.

Another example of the invention is shown in FIG- URES 5 and 5a. In this design, the pump is also meant to be mounted in the vacuum chamber. The centrallylocated cylindrical pump container, made out of nonmagnetic material, has a number of radial hollow ribs 51 through 56 and is concentrically mounted through a thinwalled tube 58 made from thermally, poor-conducting material to the cover flange 59 of the pump. Mounted in the inside of the hollow ribs are the flat coils 60 to 65 made out of superconducting wire. Their connecting leads are led to the outside through the supporting tube. When excited, these flat coils produce a strong circular magnetic tfield which can reach 5-14 kg. when using hard superconductors for the coils. The central container and the hollow ribs connected with it are filled during operation with liquid helium whose vapor can escape through tube 58. Ribs 51 through 56 make up the anode of an ion getter pump. As a cathode, the sheets '66 made from a suitable getter material such as zirconium, molybdenum or titanium are used and are supported by the electrical connectors 67. The latter are brought out through cover 59 through electrical insulation. The actual pump body with the coil containing ribs is grounded.

The design is laid out such that the gas to be pumped in the vacuum chamber can reach the cool ribs without finding any resistance and without travel loss where it will condense to a solid due to the low temperature. Due to a high DC voltage applied between anode and cathode, a weak flow discharge burns between these electrodes, evaporating the getter material of the cathode which condenses on the deep, cooled hollow ribs and forms an active getter film. This film, due to the low substrate temperature, is structurally strong and rough and has an excellent ability to absorb the gas to be pumped. As in prior examples, a continuous magnetic field can be produced which does not require power supply from the outside. To minimize outside radiation, a baflle can be used as it was in FIGURE 3 which surrounds the whole pumping system and which is held by thermal contact with the supporting tube 58 at an intermediate temperature.

I claim:

1. A high vacuum pump comprising:

a cathode electrode having a surface containing getter material;

a liquid container having an outer surface forming an anode electrode, said electrodes arranged in relatively close proximity to one another and connected to a high voltage for establishing a glow discharge therebetween;

a liquified gas charge contained in said liquid container for maintaining said anode at a very low temperature during operation;

electromagnet means establishing a magnetic field between said electrode; and

said electromagnet means is a superconductant and is disposed within said liquid container immersed in said liquified gas.

2. A high vacuum pump as claimed in claim 1 including a pair of shield plates connected to opposed ends of said container and fabricated from superconducting material, said shield plates operative to concentrate magnetic flux to the space between electrodes.

3. A high vacuum pump as claimed in claim 1 wherein: said electrodes have a plurality of segments alternately arranged in a circular path; and said electromagnetic means is comprised of a plurality of immersed superconductant coils generating a circular magnetic field passing through said alternatelyarranged electrode segments.

10 References Cited UNITED STATES PATENTS 3,117,247 1/1964 Iepsen 23069 XR 3,236,442 2/1966 Davis et al. Z3069 ROBERT M. WALKER, Primary Examiner. 

